Anharmonic suppression of Charge density wave in 2H-NbS$_2$

TL;DR

This study investigates the phonon spectrum of 2H-NbS$_2$, revealing that anharmonic effects suppress charge density wave formation despite strong phonon softening, contrasting with similar compounds.

Contribution

It demonstrates that anharmonic effects prevent charge density wave in 2H-NbS$_2$, highlighting the role of anharmonicity and anisotropic electron-phonon coupling.

Findings

Phonon softening is temperature-dependent and anisotropic.

No charge density wave occurs due to anharmonic suppression.

2H-NbS$_2$ is near the CDW transition but remains stable.

Abstract

The temperature dependence of the phonon spectrum in the superconducting transition metal dichalcogenide 2H-NbS$_2$ is measured by diffuse and inelastic x-ray scattering. A deep, wide and strongly temperature dependent softening, of the two lowest energy longitudinal phonons bands, appears along the $\mathrm{\Gamma M}$ symmetry line in reciprocal space. In sharp contrast to the iso-electronic compounds 2H-NbSe$_2$, the soft phonons energies are finite, even at very low temperature, and no charge density wave instability occurs, in disagreement with harmonic ab-initio calculations. We show that 2H-NbS$_2$ is at the verge of the charge density wave transition and its occurrence is only suppressed by the large anharmonic effects. Moreover, the anharmonicity and the electron phonon coupling both show a strong in-plane anisotropy.